Fiber reinforced glass matrix composites have, in recent years, come to replace metal in many services requiring high strength, stiffness, and light weight. Among the more common fiber reinforced glass matrix composites are carbon fiber reinforced composites. Such composites can be found in products ranging from sporting goods to jet engines.
Carbon fiber reinforced glass matrix composites (CFRGM composites) typically comprise a glass or glass-ceramic matrix in which carbon fibers are imbedded. The carbon reinforcing fibers may be either continuous or discontinuous depending on the application. Continuous fibers extend for the entire length of a composite article, while discontinuous fibers, which are significantly shorter than continuous fibers, provide more localized matrix reinforcement. As a result, continuous CFRGM composites are often used for load bearing structural applications, while discontinuous CFRGM composites are more suitable for nonload or low load bearing nonstructural applications. Discontinuous CFRGM composites are especially suitable for articles having complex shapes.
The interaction between the carbon fibers and the matrix gives CFRGM composites superior mechanical properties that make them suitable replacements for metals. The fibers contribute to the composite's strength and elastic modulus by absorbing loads transferred from the matrix through fiber-matrix interfacial bonds. The fibers also improve the composite's toughness by inhibiting or blunting the formation of cracks in the matrix. In addition, carbon fibers exposed at the surface of the matrix impart their good lubricating properties to the composite.
Despite their overall superior mechanical properties, CFRGM composites, especially injection molded discontinuous CFRGM composites, can have low Weibull moduli. The Weibull modulus is a statistical measure of the spread in mechanical property data, such as flexural strength data, for many composite samples. A lower Weibull modulus indicates a greater spread in the data. A low Weibull modulus can be undesirable because it can require engineers to apply a large design allowance to articles to ensure reliability. This can require articles to be larger and heavier than if the material had a higher Weibull modulus. As a result, materials with high Weibull moduli are desirable, especially for applications in which size, weight, and reliability are important considerations.
CFRGM composites, like other carbon-containing composites, are susceptible to carbon oxidation. This problem is exacerbated by the presence of matrix microcracks that form during fabrication as a result of a thermal expansion mismatch between the glass matrix and carbon fibers. Microcracking is especially extensive in composites with discontinuous fibers because of the complex stress states arising from the random three-dimensional arrangement of the fibers. Matrix microcracks provide channels that permit oxygen to penetrate the matrix, providing the opportunity for carbon fibers in the interior of the matrix to oxidize when exposed to elevated temperatures. Carbon fiber oxidation can quickly destroy the composite's strength and lubricity, making carbon fiber composites unsuitable for certain applications or requiring frequent replacement of parts constructed from these composites.
Accordingly it would be desirable to have a CFRGM composite that has a high Weibull modulus, improved reliability, resists oxidation, and maintains its strength and lubricity, particularly at high temperatures.